The present invention relates to a hydraulic brake system for automotive vehicles of the type including a brake master cylinder connected to an unpressurized reservoir and a brake line communicating with a working chamber of the brake master cylinder and leading to pressure control valves of a brake slip control apparatus, with at least one wheel brake connected downstream of the pressure control valves.
Brake systems of this type require during operation of the brake slip control apparatus for the repeated release and actuation of the wheel brake a quantity of pressure fluid which greatly exceeds the feed capacity of the brake master cylinder. Therefore, in these systems it is necessary during operation of the brake slip control apparatus that the wheel brake be supplied with pressure fluid out of an additional pressure fluid source, for example an energy supply system which in independent of the brake master cylinder. Further, in such systems there is the requirement in the event of failure of either the brake slip control apparatus or the energy supply system during operation of the brake slip control apparatus, the brake master cylinder be maintained in an actuating position to ensure continued actuation of the wheel brake until the vehicle is stopped.
A brake system of this type is disclosed in the German patent application No. P 34 39 258.0, wherein there is a brake power booster fed by a hydraulic energy supply system for the purpose of actuating the brake master cylinder. In this brake system, the brake line is connectible to the booster chamber of the brake power booster by way of an electromagnetically actuatable control valve of the brake slip control apparatus and a non-return valve connected down-stream thereof. This arrangement supplies the wheel brake cylinders with pressure fluid from the energy supply system of the brake power booster on operation of the brake slip control apparatus. Furthermore, this system includes a controllable non-return valve between the brake master cylinder and the control valve which non-return valve is caused by the pressure metered through the control valve to switch to its closed position providing for only return flow of pressure fluid to the brake master cylinder. As a result the brake master cylinder is kept in an alert position during operation of the brake slip control apparatus. This structure for keeping the master cylinder in an alert position is however restricted to brake systems which include a hydraulic brake power booster. Such brake systems are comparatively complicated and expensive and are not particularly applicable for use on smaller-sized vehicles. Another disadvantage of this type of brake system lies in that proper functioning of the hydraulic brake power booster and of the brake slip control apparatus depend on each other.